1. Field of the Invention
Aspects of the invention generally relate to heat transfer and, more specifically, to vapor chamber structures for efficient dissipation of heat from semiconductor devices.
2. Description of Related Art
Removal of excessive heat from electronic packages using heat sinks is known in the semiconductor industry. To meet the increasing demand of heat flux density for high-power electronic products, various heat sink designs have been used. For example, tall, thin, flat fins can provide more cooling surface area for convective heat removal, and lower pressure drop for greater airflow. Further, the heat transfer coefficient (measured in watts per surface area per degree Celsius), and the temperature gradient, can be increased by crosscutting flat fins into multiple short sections if the direction of airflow is random. The leading and trailing edges of a fin can be augmented with a curvature on the surface to scrub dead air when air velocity is high. To overcome the extrusion ratio limit, i.e. the aspect ratio of fin height to spacing, during the extrusion process, fins can be assembled and bonded to a separate base to significantly increase the cooling surface area. Corrugated metal sheet can also be used as lightweight folded fins to increase the cooling surface area.
In addition to improved fin efficiency, an ideal heat sink is desired to be made of materials that have high thermal conductivity, high machinability, low cost, low weight, and low toxicity. However, most materials that offer better properties than aluminum often are expensive to manufacture. For example, copper (390 W/m-K) offers higher thermal conductivity than aluminum (230 W/m-K), but it weighs 3 times heavier and is more difficult to machine. Graphite composite, on the other hand, is lighter but has anisotropic thermal conductivity.
Since air is a not a good agent for heat transfer due to its low specific density, low specific heat, and low thermal conductivity, alternative technologies that offer higher heat removal capacities have been proposed to replace the traditional low-cost and low-maintenance air-cooling technologies. For example, the use of liquid, typically a water and glycol mixture, instead of air not only reduces the size of the heat sink while eliminating fan noise, but also easily removes heat from its source, thereby increasing system reliability.
Heat pipes and vapor chambers are other promising technologies based on the principle of two-phase heat exchange. In a typical tubular heat pipe, where a vacuum-tight pipe having a wick structure and working fluid is used to connect an evaporator unit and a condenser unit, heat generated in the evaporator unit vaporizes the liquid in the wick. The vapor then carries the latent heat of vaporization and flows into the cooler condenser unit, where it condenses and releases the heat. The condensed liquid is returned to the evaporator unit through the capillary action of the wick structure. The phase change, caused by vaporization and condensation, and the two-phase flow circulation continue until the temperature gradient between the evaporator and the condenser no longer exists.
Compared to a solid material such as aluminum that removes heat through thermal conduction, an average heat pipe that transports latent heat through vapor flow has an equivalent thermal conductivity more than 1000 W/m-K. A pump with micro-channels may be used if the condenser unit is far from the evaporator unit. Although water can be used as the working fluid for a wide range of temperatures, other materials such as methanol, ammonia, propylene, ethane, nitrogen, oxygen, and hydrogen are more suitable for low-temperature operation. For high-temperature operations, alkali metals such as cesium, potassium, sodium, and lithium appear to be more suitable.
The passive nature and nearly isothermal heat transfer of the heat pipe technology make it attractive in many cooling and thermal control applications. More recent heat pipes have sintered wicks that could return liquid against gravity by capillary flow and provide a higher heat flux handling capability.
Similar to heat pipe, which is a closed-loop, phase-change heat transfer system, a vapor chamber provides flat plate on the surface of heat source and allows direct attachment to a heat sink. In U.S. Pat. No. 6,085,831, entitled “Direct chip-cooling through liquid vaporization heat exchange” discloses a mechanism for dissipating heat from a chip. One of the drawbacks of this proposal is the limited surface area available for cooling the vapor as the interior fins inside the heat sink enclosure are not in direct contact with the cooler ambient that is outside the heat sink. To enhance heat exchange efficiency, the enclosure must be significantly larger than the conventional air-cooled heat sink to provide the necessary heat removal capacity. Since the internal fins generally provide lower cooling efficiency than external fins, it is therefore desirable to overcome the drawbacks associated with earlier designs.